Multi-cell interference is undoubtedly the major limiting factor in full-coverage broadband wireless access networks. Mitigating its effect onto the downlink data transmission is one of the key challenges in future wireless communication systems. The better the knowledge about the interference channels, the better is also the basis for any interference mitigation.
Multi-cell channel knowledge can be exploited by simple, receiver-based schemes as interference rejection combining (IRC) (cf. C. A. Baird and C. L. Zahm, “Performance criteria for narrowband array processing,” in IEEE Conference on Decision and Control, vol. 10, 1971, pp. 564-565) so as to increase post-equalization SINR (signal-to-noise-ratio where noise includes both thermal noise and interference) of the terminal in the downlink, in particular in the LTE (long term evaluation) downlink, and thus the throughput, or advanced schemes based on cooperative base stations such as joint transmission (JT) (cf. T. Weber, I. Maniatis, A. Sklavos, Y. Liu, E. Costa, H. Haas, and E. Schulz, “Joint transmission and detection integrated network (joint), a generic proposal for beyond 3g systems,” in 9th International Conference on Telecommunications (ICT'02), Beijing, vol. 3, June 2002, pp. 479-483; H. Huang and S. Venkatesan, “Asymptotic downlink capacity of coordinated cellular networks,” in Conference Record of the Thirty-Eighth Asilomar Conference on Signals, Systems and Computers, 2004., vol. 1, 2004, pp. 850-855).
In order to estimate the channels of interferers, the base stations (base stations) must transmit pilot symbols as training signals being orthogonal among the antennas of different sectors and sites, wherein at the receiving site in the mobile terminal (MT) the received pilot symbols are compared with a reference in order to judge kind and extent of the interferences in the respective channels. On the other hand, quite a lot of these channels must be estimated to combat the interference until the noise floor is reached. The more interferer channels are distinguishable, the more orthogonal pilots must be transmitted. This consumes a large fraction of the potential capacity gain.
In particular, in the standard of the cellular system LTE, the aforementioned issue has been left open which, however, needs to be considered for a fully working system achieving higher throughput. Therefore, it is of great interest to gain knowledge of the interference channels as already mentioned above. The 3G-LTE (third generation long term evaluation) system provides built-in means to independently estimate the intra-cell channels belonging to the same Node-B based on pilots. So far, there is no efficient proposal of how to extend this to all Node-base station in such a system.
For Node-B cooperation in FDD (frequency division duplex) multi-cell applications any kind of channel state information is required at the Node-base station (coherent channel state information (CSI) or incoherent channel quality information, CQI)). This knowledge may be gained based on a multi-cell channel estimation to be performed at the terminal side based on orthogonal pilots. It is not possible to define as many orthogonal pilots as required for a reasonable estimation of the multi cellular channels since this would reduce the spectral efficiency of the system due to the pilot overhead. So it has to find some kind of a gap in the standard how to orthogonalize the base station signals.
In the prior art, the use of code-division multiplexing is known from the WCDMA (wideband code division multiple access) systems. In this system, the common downlink pilot channel (CDPICH) is spread by a cell-specific length-256 sequence, which gives an estimator gain of at most 24 dB against other signals after correlation over the sequence length. In this way, at least the strongest interference signals can be estimated at the terminal side.
There is no comparable technique available for OFDM-based cellular systems, where the channel is estimated in a frequency-selective manner, and most of the pilot resources available for interference suppression is used for accurate frequency-selective channel estimation. Spreading gains as high as in the WCDMA standard are unknown and not feasible in OFDM systems like 3G LTE.
Moreover, it has been found that it is difficult to obtain a precise downlink CSI as required for joint transmission at the base station; namely, the requirements are stricter than whose for joint detection in the opposite uplink direction (cf. T. Haustein, C. von Helmolt, E. Jorswieck, V. Jungnickel, and V. Pohl, “Performance of MIMO systems with channel inversion,” in IEEE 55th Vehicular Technology Conference, 2002. VTC Spring 2002., vol. 1, 2002, pp. 35-39). As already mentioned above, modern cellular systems may unfortunately not provide as many of orthogonal pilot symbols as need to estimate all channels independently.